The chromatin structure of genes coding for P-450 enzymes is investigated. Changes in the chromatin structure of these genes upon gene activation is examined by comparing the chromatin structure of the genes in nuclei purified from the livers of both normal and carcinogen-treated rats. Micrococcal nuclease digestion reveals that one of the genes, P-450M, is present in a non-nucleosomal conformation. Four DNase I hypersensitive sites have been mapped in this gene. The sites do not change upon gene induction and are not present in tissues, such as rat thymus, which do not express this gene. The P-450M gene seems to be associated with the nuclear matrix. Using a plasmid containing the entire cloned P-450M sequence, an S1 sensitive site has been mapped. The DNA region recognizable by S1 may also be recognized by regulatory chromosomal proteins.